Bleaching out electrophotographic sensitizers by simultaneous exposure to light and corona discharge



United States Patent Oftice 3,418,115 Patented Dec. 24, 1968 2 Claims. (c1. 96-1) ABSTRACT OF THE DISCLOSURE Dye-sensitized electrophotographic layer has the dye bleached out after image is developed by exposing the layer to dye-absorbing light in presence of corona discharge.

Photoconductive layers generally consist of a photoconductive compound finely distributed in a film-forming insulating binding agent, for instance, photoconductive ZnO dispersed in a silicone resin as film-forming binding agent.

It is known to improve the sensitivity of such layers by spectral sensitization with sensitizing dyes. Due to the presence of the dyes the sensitized layers are more or less strongly colored.

In the preparation of reproductions and copies, especially when colored pictures are concerned, this coloring is undesirable. Various methods have been proposed for the subsequent elimination of this disturbing coloring. It has been proposed inter alia that the layers, after formation of the image, should be bleached by light. However, this represents an unallowable expenditure, since more especially with highly sensitized and thus deeply colored layers, very long times are necessary to bleach the dyes. Most often the bleaching effect is insufiicient even after standing for days in sunlight. On the other hand, solvents and chemicals have been used, but as a wet process, this procedure is complicated, since care must be taken that the photoconductive layers carrying the final image are not damaged by the action of the solvents.

It is the object of the present invention to provide a simple technique to bleach uniformly and completely the residual sensitizing dye in final electrophotographic images produced in sensitized photoconductive layers.

The above object has been attained by exposing the sensitized photoconductive layers carrying the final image with light of wavelengths which correspond to the sensitization range of the sensitizing dye, and simultaneously subjecting them to a corona discharge. A complete bleaching is obtained within a short period of time, even with a relatively high concentration of the sensitizing dye in the photoconductive layer.

The process according to the invention is characterized by its great simplicity. All disadvantages of the known process are avoided. Since an intensive light source and also a corona electrode are always available in the production of electrostatic images, no additional equipment is required for the process according to the invention.

The instant bleaching process is not limited to special photoconductive layers, but is applicable to all photo conductive materials which are known in the electrophotograp'hic art.

Such materials consist of an electrically conductive support and a photoconductive layer coated thereon. As photoconductors can be used organic as well as inorganic compounds such as zinc oxide, zinc sulfide, cadmium sulfide, mercuric sulfide, lead iodide and the like.

Suitable binding agents include organic silicone resins, polyester resins such as a alkyl resins, e.g., styrenealkyd resins, silicone-alkyd resins, and oilmodified alkyd resins such as soya-alkyd resins. Useful resins are described in the book Chemie und T echnologie der Kunststolfe by R. Houwink/A. J. Staverman, in particular volume II, 2, published by Akademische Veragsgesellschaft, Leipzig, 1963. As binding agents can further be used styrenebutadiene copolymers, polyvinylchloride, polyvinylacetate and the like. Suitable as supports are, for example, metal foils or metallized plastic foils, or paper of sufficiently high electric conductivity.

The photoconductive layers are sensitized by addition of one or more of the common sensitizing dyes such as described in British patent specifications Nos. 874,133 or 919,684 or in US. Patent 3,128,179.

Suitable are, for example, the following dyes:

Rose Bengale(C.I. 45,440)

Bromophenol Blue(British Patent 874,133) 'Eosin-(C.I. 45,380)

Erythrosin-(C.I. 45,430)

Example 1 9 g. of a photoconductive zinc oxide and 50 mg. of Rose Bengale dissolved in ethyl alcohol are thoroughly mixed and dried. The zinc oxide containing dye is then ground with the solution of 0.75 g. of a phenylmethyl polysiloxane resin in 10.5 g. of toluene in a ball mill for one hour. The resulting mixture is applied onto an electrically conductive paper support and dried. The dried layer is uniformly charged and imagewise exposed.

The latent charge pattern is developed by means of a liquid aerosol as described in British patent specification No. 994,645. The final image consists of developer dye on a pink-colored background. The coloring of the layer is caused by the sensitization with Rose Bengale.

The above final image is now uniformly exposed with light which essentially consists of wavelengths which correspond to the sensitization range of Rose Bengale. Simultaneously, the image is subjected to a corona discharge, for example by treating the layer several times with one or more corona electrodes. The spacing of the corona electrode from the layer which contains the image is a few millimeters. The discharge current intensity is about 0.1 ma. The Rose Bengale is uniformly bleached (this is more clearly seen in the image-free background) and an image on a white background is obtained. Times of a few minutes are generally sufficient for the bleaching out operation.

Similar results are obtained if Rose Bengale is wholly or partially replaced by the following sensitizers: Bromophenol blue, eosine, erythrosine, fiuorescein (C.I. 45,350) or acridine orange (0.1. 46,005) or if the zinc oxide is replaced by a photoconductor such as cadmium sulfide or titanium dioxide.

If the photoconductive layer contains several sensitizers which sensitize for different regions of the spectrum, the light used for the exposure in the bleaching-out operation can be of such a composition that the sensitizers are simultaneously bleached out, i.e., it can contain light with wavelengths from the sensitization ranges of all sensitizers. If desired, the dyes can be bleached successively.

Example 2 An electrophotographic material, the photoconductive layer of which is sensitized with Rose Bengale, is produced as described in Example 1.

The dried material is cut into several samples which are processed as follows:

A first set of comparison tests comprises the exposure 3 of 9 samples with light which consists of the following wavelengths:

The exposure time was 24 hours for each sample. Samples 3, 4 and 5 showed a just noticeable bleaching effect.

In a second set of comparison tests the samples were exposed in the same manner as the samples of the first m comparison tests but were simultaneously subjected to a corona discharge. In the second set of samples the sensitizing dye is completely bleached after approximately 1 minute exposure time.

We claim:

1. In the process of bleaching away the color of the sensitizing dye from an electrophotographic image that has been developed on a surface so sensitized, which bleaching is carried out by uniformly exposing the sensitized surface to an intense light having a wavelength in the sensitizing range of the sensitizing dye, the improvement according to which the bleaching exposure is effected while the sensitized surface is subject to a uniform corona discharge.

2. The combination of claim 3 in which the sensitized surface is that of an electrically insulating layer of filmforming binding agent in which is dispersed photo-conductive zinc oxide and the sensitizing dye.

No references cited.

NORMAN G. TORCHIN, Primary Examiner.

15 J. C. COOPER, Assistant Examiner.

U.S. Cl. X.R. 96l.7, 89

@2 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,l-L18,1l5 Dated December 24, 1968 Inventor(s) Richard Menold et 8.1

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column I, line 7, should read 2. The combination of claim I in which the sensitized SIUnEU Mw SEALED DEC 2 1959 (SEAL) Atteat:

Edward M. Fletcher, It. WILLIAM E- SQHUYLER, JR.

Officer commissioner of Patents 

